The tag 'VisualStateManager.VisualStateGroups' does not exist in XML namespace 'http://schemas.microsoft.com/wpf/2008/toolkit'. Line '209' Position '26'.

I'm loading "skin" files dynamically from a filestream using the XamlReader.Load method and get the error "The tag 'VisualStateManager.VisualStateGroups' does not exist in XML namespace 'http://schemas.microsoft.com/wpf/2008/toolkit'.
Line '209' Position '26'." when it encounters the first reference to VisualStateManager.VisualStateGroups in the xaml. I have a xmlns defined ("xmlns:WPFToolkit=http://schemas.microsoft.com/wpf/2008/toolkit")
and I've tried it both with and without the WPFToolkit: prefix with the same results. The odd thing was that when we were loading the skin files via the
resources from app.xaml it all worked. Am I missing something here?